U.S. patent number 7,385,754 [Application Number 11/367,595] was granted by the patent office on 2008-06-10 for efficient wavelength referencing in a combined optical amplifier-optical channel monitor apparatus.
This patent grant is currently assigned to REDC Optical Networks Inc.. Invention is credited to Tomer Eliyahu, Uri Ghera, David Menashe, Alex Shlifer.
United States Patent |
7,385,754 |
Ghera , et al. |
June 10, 2008 |
Efficient wavelength referencing in a combined optical
amplifier-optical channel monitor apparatus
Abstract
Efficient wavelength calibration in a WDM optical amplifier that
includes an optical channel monitor (OCM) is obtained by
introducing a notch into the amplified spontaneous emission (ASE)
noise spectrum of the amplifier outside a WDM transmission band,
and having the OCM detect the notch and use the notch as a
reference to calibrate the wavelength measurement. The notch is
introduced into the ASE noise spectrum using a notch filter, which
is preferably incorporated in a gain flattening filter (GFF).
Inventors: |
Ghera; Uri (Tel-Aviv,
IL), Shlifer; Alex (Rehovot, IL), Eliyahu;
Tomer (Rishon Lezion, IL), Menashe; David
(Tel-Aviv, IL) |
Assignee: |
REDC Optical Networks Inc.
(Tel-Aviv, IL)
|
Family
ID: |
38471212 |
Appl.
No.: |
11/367,595 |
Filed: |
March 6, 2006 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20070206273 A1 |
Sep 6, 2007 |
|
Current U.S.
Class: |
359/337.1;
398/149 |
Current CPC
Class: |
H04J
14/02 (20130101); H04J 14/0221 (20130101) |
Current International
Class: |
H01S
3/00 (20060101) |
Field of
Search: |
;359/333,337.1
;398/149 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Bolda; Eric
Attorney, Agent or Firm: Friedman, Ltd.; Mark M.
Claims
What is claimed is:
1. An optical amplifier comprising: a. at least one gain stage for
amplifying an incoming optical signal within a required
transmission band, the amplified signal including an amplified
spontaneous emission (ASE) noise spectrum; b. at least one notch
filter for introducing two notches into the ASE noise spectrum
bordering externally the required transmission band, wherein the at
least one notch filter is combined with a gain flattening filter
(GFF) in a single component; and c. at least one optical channel
monitor (OCM) configured to provide wavelength referencing based on
the two notches; whereby the optical amplifier does not include any
external light source.
2. The optical amplifier of claim 1, wherein the two notches are
each characterized by a narrow region of high attenuation in the
ASE noise spectrum.
3. The optical amplifier of claim 1, wherein the transmission band
is a C-Band having two edges, and wherein the two notches border
the C-band at approximately 1 nm from each edge.
4. The optical amplifier of claim 1, wherein the at least one gain
stage includes at least two gain stages and wherein the at least
one notch filter combined with a GFF in a single component is
positioned between two gain stages.
5. The optical amplifier of claim 4, further comprising a mid-stage
access element positioned between two gain stages and operative to
provide connectivity to external devices.
6. The optical amplifier of claim 5, further comprising a tap and a
switch for allowing alternate monitoring of the optical signal both
before the mid-stage access element and at an optical amplifier
output.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to optical communication systems, and
more particularly to optical amplifiers incorporating an Optical
Channel Monitor (OCM) used in Wavelength Division Multiplexing
(WDM) optical communication systems.
Optical amplifiers, such as Erbium Doped Fiber Amplifiers (EDFAs)
and Raman amplifiers, are used in WDM optical communication systems
for amplifying many wavelength channels simultaneously in the
transmission band. Modem WDM systems also employ Dynamic Gain
Equalizers (DGEs), Reconfigurable Optical Add Drop Multiplexing
(ROADM) and Optical Cross Connects to manipulate individual
wavelength channels as they are transmitted along the system. For
example, DGEs are able to control the power of individual channels
in order to ensure that all channels have the same optical power
along the system. A ROADM module allows dynamic and reconfigurable
selection of which wavelength channels are to be added or dropped
at intermediate nodes of the system.
Since these wavelength manipulation devices introduce additional
insertion loss into the system, they are often located at the
mid-stage of double or multi-stage amplifiers typically found at
each network node. This reduces the impact of the device insertion
loss on system performance. Thus, the optical amplifier found at
each network node often forms the heart of a local optical
sub-system, including the amplifier itself and the various
wavelength manipulation devices incorporated in the amplifier
mid-stage.
Due to the dynamic nature of these systems and the fact that
manipulation occurs at the individual channel level, OCMs are
necessary to monitor the wavelength, optical power and Optical
Signal to Noise Ratio (OSNR) of each channel. Typically, OCMs are
located at each network node, together with the optical amplifiers
and wavelength manipulation devices, and are used to provide the
system management software with a full spectral picture of the
system at all times.
In current network implementations, the OCM is typically located on
a separate card occupying its own slot within the network rack. A
much improved solution would be to integrate the OCM within the
optical amplifier, thus reducing overall cost and also reducing
space requirements by freeing up an extra slot. Furthermore,
integrating the OCM within the amplifier would allow the amplifier
itself to directly make use of the information provided by the OCM
without mediation of the system management software. For example,
the OCM can be used to fine tune the automatic gain control (AGC)
of the amplifier in response to evolving spectral conditions
(different channel loading conditions at the amplifier input).
Furthermore, the OCM can be used by the amplifier to provide local
management to the spectral manipulation devices located at the
amplifier mid-stage. For example, the amplifier could use the OCM
to check whether the channels comprising the output WDM signal have
equal power, and if not provide feed-back to a DGE to achieve the
required equalization.
An integrated OCM within an optical amplifier should be cost
affective and have a small footprint so as not to occupy too much
space on the amplifier printed circuit board (PCB). One
particularly attractive option for implementing such an OCM is to
use a tamable optical filter together with an optical detector.
Such a filter transmits or reflects only a narrow portion of the
transmission band, which can then be detected by the detector. By
scanning the filter across the entire transmission band, a full
spectral picture of the transmission band can be obtained, and the
channel information (wavelength, power, OSNR) can be extracted
using suitable signal processing methods. Tunable filters can be
implemented using for example thin film technology to create a
Fabry-Perot type filter, with the tuning accomplished either by
temperature effects, using liquid crystals, or by tuning the angle
at which the signal is incident on the filter. Other technologies
for implementing tunable filters include, but are not limited to,
tunable Bragg gratings and filters based on the acousto-optic
effect.
A key requirement of all tunable filters is to be able to calibrate
the wavelength setting of the filter. In a typical OCM application
the filter will be continuously scanned across the transmission
band during the entire lifetime of the system, necessarily causing
some aging effect. This means that the wavelength settings of the
filter may change with time. For example, if the center wavelength
of the tunable filter is determined by an applied voltage, then the
relation between the voltage magnitude and center wavelength will
change with time. This means that if the filter is only calibrated
at the production stage, then the channel wavelengths calculated
and reported by the signal processing algorithms will drift with
time and eventually cease to be accurate. To overcome this problem,
continuous and real-time calibration is required to maintain
accurate operation of the OCM throughout its lifetime.
One method to achieve such calibration employs a separate reference
signal which is outside the required transmission band, and which
is multiplexed with the main signal to provide a fixed reference
point for the tunable filter. Such a method is described for
example in U.S. Pat. No. 6,473,234 to Kuznetsov, and U.S. Pat. No.
6,509,972 to Korn. Both patents disclose the use of a broad band
light source together with a fixed narrow line-width Fabry-Perot
filter to provide a stable narrow reference signal for the tunable
filter. U.S. Pat. No. 6,619,864 to Johnson et al discloses a
similar method where the fixed narrow line-width Fabry-Perot filter
is replaced with a gas cell providing narrow well defined
absorption lines. U.S. Pat. No. 6,619,864 to Althouse et al
describes a method where the reference signal is within the
transmission band, but an optical switch is used to alternate the
input to the tunable filter between the reference signal and the
signal to be measured.
All the referencing methods described above require additional
optical components, thus increasing the overall power and space
requirements and increasing cost. Furthermore, the additional
components introduce added complexity and points of failure within
the system, thus reducing overall reliability. Thus, while they may
be suitable for stand-alone OCMs, they are unsuitable for OCMs
integrated within optical amplifiers. Therefore, there is a need
for an optical channel monitor within (or combined with) an optical
amplifier, which allows continuous and real-time wavelength
calibration with minimum additional components and complexity.
SUMMARY OF THE INVENTION
The present invention relates to efficient wavelength calibration
in an integrated or combined optical amplifier--optical channel
monitor (OCM) apparatus. The efficient wavelength calibration is
obtained by introducing a "notch" or notches into the amplifier
amplified spontaneous emission (ASE) noise spectrum outside the WDM
transmission band. Each notch is preferably a very narrow, well
defined, highly attenuated region within the ASE noise spectrum.
Alternatively, the notch may be a very narrow non-attenuated region
in an otherwise wholly attenuated ASE noise spectrum outside the
transmission band. Each notch is detected by the OCM and used as a
reference to calibrate the wavelength measurement. A notch is
introduced into the ASE noise spectrum using a notch filter, which
is preferably incorporated in (or alternatively combined with) a
gain flattening filter (GFF). This provides an easy and efficient
wavelength reference.
To emphasize, the OCM detects the presence of at least one notch
within the background ASE noise spectrum and uses the known
wavelength position of each notch to calibrate the measured
wavelength of the transmission channels. The concept of introducing
a notch in the ASE noise spectrum and using for wavelength
calibration is unique to the present invention.
In a preferred embodiment, the notch filter is part of a gain
flattening filter (GFF). In alternative embodiments, the notch
filter and gain flattening filter are separate.
In some embodiments, the OCM and the notch filter may be positioned
externally to the optical amplifier and used with its generated ASE
noise to provide easy referencing. Alternatively, an OCM, optical
amplifier and notch filter may be integrated in a system in any
combination or arrangement that provides the referencing described
in detail below.
Advantageously, in "embedded" OCM embodiments in which the OCM and
notch filter are integrated in the optical amplifier, the
integration increases the system efficiency because it removes the
need for additional components whose only purpose is to provide a
wavelength reference. Thus, the cost, complexity and space
requirements of the integration are significantly reduced.
According to the present invention there is provided an optical
amplifier comprising at least one gain stage for amplifying an
incoming optical signal within a required transmission band, the
amplified signal including an ASE noise spectrum, at least one
notch filter for introducing at least one notch into the ASE noise
spectrum in the required transmission band; and at least one OCM
operative to provide wavelength referencing based on the at least
one notch.
In some embodiments of the optical amplifier of the present
invention, the at least one notch filter is included in a GFF.
In some embodiments of the optical amplifier of the present
invention, the at least one notch is characterized by a narrow
region of high attenuation in the ASE noise spectrum.
In some embodiments of the optical amplifier of the present
invention, the at least one notch includes two notches bordering
externally the transmission band.
In some embodiments of the optical amplifier of the present
invention, the transmission band is the C-Band and the two notches
border the C-band at approximately 1 nm from each edge of the
band.
In some embodiments, the optical amplifier includes at least two
gain stages and the GFF is positioned between two of the gain
stages.
In some embodiments of the optical amplifier of the present
invention, each gain stage includes a section of Erbium doped fiber
(EDF) and the optical amplifier further comprises at least one pump
operative to output pump energy for pumping each EDF section and at
least one WDM combiner for combining the pump energy with the
optical signal.
In some embodiments, the optical amplifier further comprises a
mid-stage access element positioned between two gain stages and
operative to provide connectivity to external devices.
In some embodiments, the optical amplifier further comprises an
additional tap and a switch for allowing alternate monitoring of
the optical signal both before the mid-stage access element and at
an optical amplifier output.
According to the present invention there is provided an optical
communication system comprising a WDM transmitter operative to
transmit at least one WDM signal channel in a required transmission
band, at least one optical amplifier operative to amplify each
signal channel, the amplifier having an output that includes an
amplified spontaneous emission (ASE) noise spectrum, at least one
notch filter operative to introduce at least one notch into the ASE
noise spectrum, at least one OCM operative to provide wavelength
referencing based on the at least one notch and a WDM receiver to
receive the at least one WDM signal channel
In some embodiments of the optical communication system of the
present invention, the at least one notch filter is included in a
gain flattening filter (GFF).
In some embodiments of the optical communication system of the
present invention, the at least one notch filter and the OCM are
integrated into a single unit.
In some embodiments of the optical communication system of the
present invention, the at least one notch is a narrow region of
high attenuation in the ASE noise spectrum
In some embodiments of the optical communication system of the
present invention, the at least one notch includes a single notch
bordering the transmission band on one band side.
In some embodiments of the optical communication system of the
present invention, the at least one notch includes two notches
bordering the transmission band on opposite band sides.
In some embodiments of the optical communication system of the
present invention, the transmission band is a C-Band and each notch
is a narrow high attenuation region located approximately 1 nm from
the respective edge of the C-Band.
In some embodiments of the optical communication system of the
present invention, the notch filter and the OCM are integrated in
the optical amplifier.
According to the present invention there is provided a method for
calibrating the wavelength measurement of an OCM in a WDM optical
communication system, the method comprising the steps of obtaining
an amplified optical signal that includes an ASE noise spectrum,
filtering the amplified optical signal to introduce at least one
notch in the ASE noise spectrum outside a required transmission
band, and operating the OCM to detect the at least one notch and to
use the detected notch to calibrate the wavelength measurement of
the OCM
In some embodiments of the method of the present invention, the
introducing of at least one notch includes introducing a narrow
region of high attenuation in the ASE noise spectrum.
In some embodiments of the method of the present invention, the
introducing of the at least one notch includes performing an action
selected from the group of introducing at least one notch bordering
the transmission band on one band side and introducing two notches
bordering the transmission band on opposite band sides.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, by way of example only, with
reference to the accompanying drawings, wherein:
FIG. 1a describes an optical amplifier with an embedded OCM
according to the preferred embodiment of the invention;
FIG. 1b shows an exemplary construction of the OCM used in the
preferred embodiment of the invention;
FIG. 2 shows an exemplary shape of the notch filter used in the
preferred embodiment of the invention;
FIG. 3 shows an exemplary Optical Spectrum Analyzer (OSA) trace
taken at the output of the notch filter;
FIG. 4 shows an exemplary signal produced by the OCM, including the
notch that is used for wavelength calibration of the OCM;
FIG. 5 shows an exemplary method by which wavelength calibration of
the OCM is performed;
FIG. 6 describes an optical amplifier with an embedded OCM
according to another embodiment of the invention;
FIG. 7 describes an optical amplifier with an embedded OCM
according to yet another embodiment of the invention;
FIG. 8 describes an optical amplifier with an embedded OCM
according to yet another embodiment of the invention;
FIG. 9 describes an optical amplifier with an embedded OCM
according to yet another embodiment of the invention;
FIG. 10 describes an optical communication system including an
optical amplifier and OCM according to yet another embodiment of
the invention;
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The principles and operation of the apparatus of the present
invention may be better understood with reference to the drawings
and the accompanying description.
Referring now to the drawings, FIG. 1a shows an optical amplifier
unit 100 with an embedded OCM according to the preferred embodiment
of the invention. Unit 100 includes a gain stage 102, followed by a
notch filter 104 and then an optical tap 106 which diverts a small
fraction of the optical signal to an OCM 108. In operation, a WDM
signal comprising at least one data channel contained within a
transmission band (for example, the C-Band, between 1525 and
1565mn) enters unit 100 at a unit input 110. The WDM signal is then
amplified by gain stage 102, which also introduces ASE noise within
and outside the transmission band. The amplified signal and ASE
noise then pass through notch filter 104, which introduces at least
one notch in the ASE noise spectrum outside the transmission band.
A small fraction of the combined WDM signal and filtered ASE noise
spectrum is then diverted by tap 106 and is detected by OCM 108.
The combined spectrum is analyzed by the OCM 108, and the notches
within the spectrum are identified. Using the known and stored
wavelength positions of each notch, the OCM then is able to
calibrate the wavelength of all other features within the detected
signal, and in particular the wavelengths of the WDM signal
channels.
Gain stage 102 may be any type of optical amplifier, such as an
erbium doped fiber amplifier (EDFA), Semiconductor Optical
Amplifier (SOA), or Raman amplifier. Amplifier Unit 100 may include
additional gain stages (not shown), which may or may not be of the
same type as gain stage 102. These additional gain stages may
precede or follow notch filter 104.
FIG. 1b shows an exemplary implementation of OCM 108, which
includes a tunable transmission filter 114, an optical detector 116
and a processing and control unit 118. In operation, processing and
control unit 118 scans tunable filter 114 across a wavelength band
containing within it the WDM transmission band, and also a region
adjacent to the transmission band which contains the features
(notches) of notch filter 104. The scan operation may be achieved,
for example, by changing an applied voltage to the tunable filter.
The signal detected by optical detector 116 during the scan is
recorded by processing unit 118 and represents the optical power as
a function of the wavelength position of the tunable filter at any
given time during the scan. Unit 118 may also be part of the
overall processing and control unit of amplifier unit 100 (i.e.
unit 118 may perform additional functions needed to control the
other components of amplifier unit 100).
OCM 108 may also be implemented with a tunable reflection filter,
in which case the tunable filter reflects a narrow pass-band
portion of the signal, which is then detected by an optical
detector. OCM 108 may also be implemented using other methods known
to the art in order to provide a spectral analysis of the signal,
for example, using a wavelength spreading device (such as a
grating), followed by an array of optical detectors positioned to
detect different regions of the spectrum.
FIG. 2 shows an exemplary shape 200 of notch filter 104,
represented by the response of the filter (e.g. transmission) as a
function of wavelength. Within a WDM transmission band 202, the
response of the filter is basically flat, so as not to adversely
affect the WDM signals. Alternatively, the response of the filter
within the transmission band may have a specific pre-determined
shape used for example to compensate for the spectral gain shape of
gain stage 102. In this case, notch filter 104 also functions as a
gain flattening filter (GFF). Outside the WDM transmission band,
notch filter 104 includes at least one feature, such as a notch 204
occurring before the transmission band (i.e. at shorter
wavelengths) or a notch 206 occurring after the transmission band
(i.e. at longer wavelengths). All features (e.g. notches 204 and
206) need to be contained within an OCM scan band 208 (which is the
wavelength band covered by OCM 108), in order to serve the purpose
of wavelength reference for the OCM. In the present example,
notches 204 and 206 represent very narrow, sharply attenuated
regions of the wavelength spectrum. These regions should be narrow
enough to enable OCM 108 to use them to achieve wavelength
calibration with the desired wavelength accuracy. For example, if
the desired wavelength accuracy of OCM 108 is 0.2 nm, then notches
204 and 206 should be specifically narrower than 0.2 nm,
Preferably, the response of the filter at the notch center
wavelength should be attenuated by at least 20dB relative to the
response level outside a region of width 0.2 nm around the notch
center wavelength. The positions (center wavelength) of the notches
are preferably chosen to be as close to the transmission band as
possible (and always inside OCM scan band 208), but still outside
it so as not to adversely affect signals within the transmission
band. For example, if the transmission band is 1529-1562 nm, then
notch 204 may be positioned at 1528 nm, while notch 206 may be
positioned at 1563un.
While exemplary notch filter shape 200 includes two notches 204 and
206, other exemplary shapes may include only either one of the
notches, or else may include additional notches. For example, if
WDM transmission band 202 consists of two sub-bands separated by a
vacant band, then a notch may be located within the vacant band
without adversely affecting the WDM signals. In general, the number
of notches required depends on the characteristics of OCM 108. In
some cases only a single notch at the beginning or end of
transmission band 202 will be sufficient to be used as a wavelength
reference for the OCM over the lifetime of the OCM. In other cases,
for example where the scan rate of the OCM may change with time,
two notches will be required. In still other cases, for example
when the dependence of OCM center wavelength is strongly non-linear
with scan position, more than two notches may be required.
FIG. 3 shows an exemplary Optical Spectrum Analyzer (OSA) trace 300
at output 112 of amplifier unit 100 (FIG. 1). In this example, the
transmission band 202 is between 1529-1562 nm, and notch filter 104
(FIG. 1) includes only a single notch 204 located at about 1528 nm.
WDM signal channels 302 are clearly visible on trace 300, as is an
ASE background 304 generated by gain stage 102 (FIG. 1). Also
clearly visible is a notch 306 in the ASE background, located at
about 1528 nm and corresponding to notch 204 imposed on the
spectrum by notch filter 104.
FIG. 4 shows an exemplary signal 400 generated by OCM 108, which
corresponds to exemplary OSA trace 300 shown in FIG. 3. A raw
signal 402 generated by the OCM may be quite noisy, however, this
noise may be reduced by appropriate signal processing (for example,
in this case a ruing average of 10 sample points) to produce a
clean signal 404. This clean signal clearly shows a notch 406,
which corresponds to notch 306 on OSA trace 300. Note that in this
example notch 406 is smeared compared to original notch 204
introduced by notch filter 104. The reason for this is that the
line-width of tunable filter 114 within the OCM is large in
comparison to the width of notch 204, and the noise floor of
optical detector 116 within the OCM is such that the smaller powers
(compared to the minimum power of notch 406) cannot be detected.
Even under these conditions, notch 406 can be clearly identified by
standard signal processing algorithms, and thus serve as a
wavelength reference.
FIG. 5 shows an exemplary method 500 by which wavelength
calibration of the OCM is performed. In step 502, the OCM is
scanned across a wavelength band which contains within it the
transmission band. As discussed above with reference to FIG. 2,
notch filter 104 is designed such that notches 204 and 206 are
contained within the OCM scan band. At various points within the
scan band, the optical power is sampled and stored and constitutes
the OCM output signal. In an optional step 504, the OCM output
signal is processed to reduce excess noise. This step is needed
only when the output signal is noisy, in which case step 504
enhances the resolution of the OCM. In step 506, the notches
introduced by notch filter 104 are identified and compared with a
stored table of notches found within processing and control unit
118. Step 508 uses the stored information to retrieve the
wavelength of each of the notches, which are then used to calibrate
the wavelength of each of the sample points in the output signal of
the OCM. In step 510, the entire OCM output signal is analyzed to
identify the wavelength, power and optionally the Optical Signal to
Noise Ratio (OSNR) of each of the WDM signal channels. Note that,
depending on the particular characteristics of OCM 108, steps 504,
506 and 508 need not necessarily be carried out every time the OCM
is scanned. For example, if the OCM calibration is stable over a
long period of time (e.g. days), then steps 504, 506 and 508 need
only be carried out with a frequency sufficient to keep the OCM
accurately calibrated.
FIG. 6 describes an optical amplifier with an embedded OCM
according to another embodiment of the present invention. Elements
600-612 of this embodiment are respectively identical with
corresponding elements 100-112 of FIG. 1, except that element 604
incorporates a notch filter together with a Gain Flattening Filter
(GFF). Gain flattening filters are often found in optical
amplifiers and are used to compensate the shape of the gain
spectrum of the gain stages (such as gain stage 602) within the
amplifier. For example, a gain stage comprising an Erbium Doped
Fiber (EDF) will typically have higher gain in the short wavelength
region of the C-Band (around 1535 nm) and lower gain at higher
wavelengths. In such a case, the GFF in element 604 will be
designed to have higher attenuation at the shorter wavelengths to
compensate the higher gain, with the overall result being a flat
spectrum. The same technology used to produce the spectral shape of
the GFF (such as Bragg grating technology or thin film technology)
can also be used to introduce the notches required by the notch
filter. Thus both functions (GFF and notch filter) can be combined
within a single component, leading to savings in cost, space, as
well as reducing the overall insertion loss.
FIG. 7 describes an optical amplifier with an embedded OCM
according to yet another embodiment of the present invention.
Elements 700-712 of this embodiment are respectively identical with
corresponding elements 600-612 of FIG. 6. An added element 714
represents a second gain stage of amplifier unit 700. Second gain
stage 714 may be used for example to boost the output power of the
amplifier following the insertion loss introduced by combined GFF
and notch filter 704. Gain stage 714 may be of the same type as
gain stage 702, or of a different type. Other optical components
(not shown) may also be placed adjacent to combined GFF and notch
filter 704. For example, a variable optical attenuator (VOA) may be
utilized to operate amplifier unit 700 as a variable gain
amplifier. Additional gain stages (not shown) may be placed before
or after combined GFF and notch filter 704.
FIG. 8 describes an optical amplifier with an embedded OCM
according to yet another embodiment of the present invention.
Elements 800-814 of this embodiment are respectively identical with
corresponding elements 700-714 of FIG. 7, with the exception that
elements 802 and 814 now represent passive EDF fibers, and not full
gain stages. An added pump 816 coupled to the optical line through
a WDM combiner 818 provides pump energy to EDF fibers 802 and 814,
thus enabling optical amplification so that these fibers operate as
gain stages. EDFAs comprising EDF fibers, pumps, WDM combiners and
one or more GFFs are well known to the art. In this embodiment, EDF
fibers 802 and 814 are pumped by the same pump 816. However, they
may be pumped by separate pumps, or a splitter (not shown) may be
utilized to specifically direct a pre-determined fraction of the
pump power to each of the two fibers. Additional lengths of EDF
fiber (not shown) may also be utilized, each with their own pumps
or sharing pump power with the other EDF fibers. Other optical
components (not shown) may also be placed adjacent to element 704.
For example, a VOA may be utilized to operate amplifier unit 700 as
a variable gain EDFA,
FIG. 9 describes an optical amplifier with an embedded OCM
according to yet another embodiment of the present invention.
Elements 900-914 of this embodiment are respectively identical with
corresponding elements 700-714 of FIG. 7. An additional element 916
allows mid stage access to the amplifier unit, so that additional
devices, such as ROADM modules, DGE modules, and Dispersion
Compensating Modules (DCMs) may be connected to the host
communication system via amplifier unit 900. In many cases it is
desirable to be able to monitor the optical spectrum both before
mid-stage access (following the first gain stage) and after
mid-stage access. This is particularly relevant when active
spectrum manipulating devices such as ROADM modules and DGE modules
are connected at mid-stage. To accomplish this, an additional
optical tap 918 is placed before mid-stage and directs a small
fraction of the optical signal to an optical switch 920. Optical
switch 920 selects between the optical signal coming from tap 918
(before mid-stage) and that coming from tap 906 (at the amplifier
output). The selected signal is then directed to OCM 908 for
analysis.
In operation, switch 920 is alternately switched between the two
inputs (i.e. the optical signals coming from tap 918 and tap 906)
so that both the signal before mid-stage and the output signal of
the amplifier are continuously monitored. It is important to note
that notch filter 904 is placed before tap 918 so that all signals
reaching the OCM contain the notch in the ASE noise spectrum
required for providing the wavelength reference. In alternative
embodiments, switch 920 may have other inputs so that other optical
signals (possibly from outside amplifier unit 900) can also be
monitored by OCM 908. Alternatively yet, tap 918 may be connected
directly to OCM 908, in which case switch 920 is not required and
OCM 908 is used only to monitor the signal output to the mid-stage.
Alternatively yet, additional OCMs (not shown) may be utilized,
each OCM designated to monitor a specific optical signal.
FIG. 10 describes an optical communication system 1000 including an
optical amplifier and OCM according to yet another embodiment of
the present invention. Communication system 1000 includes a WDM
signal transmitter 1010 and signal receiver 1012 and a transmission
fiber 1014. Elements 1002 to 1008 are analogous to elements 102 to
108 of FIG. 1. In operation, transmitter 1010 generates the WDM
signal, which is amplified by optical amplifier 1002, transmitted
over transmission fiber 1014 and received by receiver 1012. As in
previous embodiments, notch filter 1004 introduces features
(notches) in the ASE noise spectrum generated by amplifier 1002,
which is outside the WDM transmission signal band. These notches
are used by OCM 1008 as wavelength references for calibration.
Additional transmission fiber and optical amplifiers (not shown)
may be utilized as needed along the communication system. In an
alternative embodiment notch, filter 1014 may be placed between tap
1006 and OCM 1008. In this way, the insertion loss of the notch
filter does not affect the transmission signal and the notch filter
becomes a part of the OCM unit.
All patents mentioned in this specification are herein incorporated
in their entirety by reference into the specification, to the same
extent as if each individual patent was specifically and
individually indicated to be incorporated herein by reference.
While the invention has been described with respect to a limited
number of embodiments, it will be appreciated that many variations,
modifications and other applications of the invention may be
made.
* * * * *